This seems to be the first example seen in the wild of a sexual escapade between a mammal and a different kind of vertebrate such as a bird, reptile or fish, "although some mammals are known to have attempted sexual relief with inanimate — including dead things — objects," said researcher Nico de Bruyn, a mammal ecologist at the University of Pretoria in South Africa.

One summer morning, scientists observing elephant seals on a beach on Marion Island near the Antarctic spotted a young male Antarctic fur seal subduing a king penguin.

"At first we thought it was hunting the penguin, but then it became clear that his intentions were rather more amorous," de Bruyn recalled today via email.

The roughly 240-pound seal subdued the 30-pound adult penguin by lying on it. The hapless bird of unknown sex struggled, rapidly flapping its flippers and attempting to stand and flee, without luck.

The seal then alternated between resting on the penguin and thrusting its pelvis at the bird in vain attempts to insert its penis for 45 minutes. Natural, unsuccessful sexual escapades by this variety of seal with members of its own species may last as long as this penguin assault did, "but yes, it is quite a long time and thus unusual," de Bruyn told LiveScience.

The seal then abruptly gave up, moving to sea and completely ignoring the target of its affections. The penguin apparently did not suffer any injury. The scientists detailed their findings in the May issue of the Journal of Ethology.

Many species perform some form of sexual harassment on members of their own species, "for a variety of reasons many of which are hotly debated," he added.

Many species of seal are polygynous, where one male mates with many females. The males often fight each other to control females.

"This system thus promotes extreme aggression in males towards each other, and if a male cannot control a beach, this aggression may spill over to sexual aggression directed at outlying females, pups or even in rare cases other seal species," de Bruyn said.

And this sexual aggression apparently might leap well beyond the species gap.

The Antarctic fur seals of Marion Island are the only seals known to eat king penguins. The thrill of the hunt felt by the seal the researchers saw may have channeled into its sex drive, as the mating season had just come to an end.

"It may have wanted to eat it and half-way through the chase changed its mind," de Bruyn speculated. "I personally believe the link between aggressive and sexual behavior is evolutionarily far closer linked than we currently believe. This has obvious implications for humans."

On the other hand, the amorous seal may simply have been sexually inexperienced and playful, and wanted practice, the researchers conjectured.

"There are many things that we do not understand about ourselves that are mirrored in other species," de Bruyn said. "Thus by continuing with research efforts on other vertebrates we could learn a great deal about the whys behind human behaviors."